Methods for trimming electrical parameters in an electrical circuit

ABSTRACT

Electrical circuit trimming methods. In one aspect of the invention, a trimming method includes assembling one or more components of an electrical circuit onto a printed circuit board having one or more electrical connections coupled to the said one or more components. An electrical parameter of the electrical circuit is then trimmed. The trimming of the electrical parameter of the electrical circuit includes removing a portion of the printed circuit board to break the electrical connection on the printed circuit board. In another aspect of the invention, the trimming the electrical parameter of the electrical circuit includes electrical programming of the electrical circuit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to electrical circuitsand, more specifically, the present invention relates to electricalcircuit trimming.

[0003] 2. Background Information

[0004] In electrical circuit manufacture, it is often desirable to trimthe circuit operation after the circuit is assembled in order tocompensate for manufacturing tolerances. Known techniques to achievetrimming of electrical parameters at final assembly include the use oftrimming potentiometers which add circuitry, are prone to instability(under mechanical/thermal stress) and also require human interaction toprovide the adjustment. Other techniques include the use of wire links(jumpers) or resistors assembled onto the printed circuit board andsubsequently disconnected (or clipped) to trim circuit parameters. Thesetechniques add cost and complexity since they often require humaninteraction to provide the trim and add components to the electricalcircuit.

[0005] Known power supplies employ sensing of the output voltage and orcurrent of the power supply to provide a feedback signal. The feedbacksignal is then used by a switching regulator to accurately control theoutput characteristic. Typically, such power supplies employ directoutput voltage and current sensing. Other configurations employ anindirect sensing of the output, often using the windings of thetransformer to derive information about the output. The indirect sensingtechniques are sometimes attractive as they eliminate some circuitry.However, indirect sensing techniques typically suffer from poorervoltage and current regulation accuracy since the feedback informationis influenced by other factors such as transformer manufacturingtolerances, which are difficult and expensive to improve.

SUMMARY OF THE INVENTION

[0006] Electrical circuit trimming methods are disclosed. In one aspectof the invention, a trimming method includes assembling one or morecomponents of an electrical circuit onto a printed circuit board havingone or more electrical connections coupled to the said one or morecomponents. An electrical parameter of the electrical circuit is thentrimmed. The trimming of the electrical parameter of the electricalcircuit includes removing a portion of the printed circuit board tobreak the electrical connection on the printed circuit board. Additionalfeatures and benefits of the present invention will become apparent fromthe detailed description, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention detailed illustrated by way of example andnot limitation in the accompanying figures.

[0008]FIG. 1 is a diagram illustrating one embodiment of the use of aslot, nibble or punched hole to break a metal printed circuit boardtrace in accordance with the teachings of the present invention.

[0009]FIG. 2 is a diagram illustrating one embodiment of the breakingoff of a piece of a printed circuit board in order to break a metalprinted circuit board trace in accordance with the teachings of thepresent invention.

[0010]FIG. 3 is a diagram illustrating one embodiment of the use a punchto remove a section of the printed circuit board to break a metalprinted circuit board trace in accordance with the teachings of thepresent invention.

[0011]FIG. 4 is a diagram illustrating one embodiment with an electricalcircuit having a plurality of trim pins in accordance with the teachingsof the present invention.

[0012]FIG. 5 is a block diagram illustrating one embodiment of a powersupply regulator in accordance with the teachings of the presentinvention.

[0013]FIG. 6 is a schematic illustrating one embodiment of a powersupply circuit including one embodiment of a power supply regulator inaccordance with the teachings of the present invention.

[0014]FIG. 7 is a diagram illustrating one embodiment of outputcharacteristic adjustment with various trim options in accordance withthe teachings of the present invention.

DETAILED DESCRIPTION

[0015] Embodiments of methods and apparatuses for trimming an electricalcircuit such as a power supply regulator are disclosed. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention. In otherinstances, well-known materials or methods have not been described indetail in order to avoid obscuring the present invention.

[0016] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner inone or more embodiments.

[0017] As an overview, embodiments of the present invention introduceseveral techniques to provide automated trimming capability in anassembled electrical circuit without adding any additional components.As such, electrical circuit parameters are trimmed in accordance withthe teachings of the present invention without adding cost to thecircuit manufacture. In one embodiment, this allows the outputcharacteristic of a switching power supply to be adjusted by trimmingspecific parameters of the switching regulator after the power supplycircuit has been assembled. This technique can be used to compensate formanufacturing tolerances in order to set the power supply outputcharacteristic accurately without the need to directly sense the output.The parameters trimmed allow easy control of output current and voltagecharacteristics.

[0018] As will be discussed, trimming can be achieved by eithermechanical or electrical techniques in accordance with the teachings ofthe present invention. For instance, mechanical trimming can be achievedby cutting metal traces on a printed circuit board by for examplesawing, nibbling or punching the printed circuit board. Electricaltrimming can be achieved by for example applying a specific voltage andcurrent combination to trim a parameter in the electrical circuit usingstandard programming techniques such as zener zapping (anti-fuse) ormetal zapping (fuse).

[0019] For explanation purposes, it is noted that specific embodimentsfor trimming a power supply regulator or a switching power supply aredescribed in detail herewith to adjust electrical parameters such as forexample the output voltage and or current of a switching power supply.However, it is appreciated that the novel trimming techniques describedherewith may also apply to other electrical circuit technologies inaccordance with the teachings of the present invention.

[0020] In one embodiment, trimming of a power supply circuit isperformed after the complete power supply circuit has been assembled. Asa result, manufacturing tolerances of all circuit components can becompensated for after assembly by trimming one or more electricalparameters. The output characteristic can therefore be measured beforeany trimming is carried out. Once the output characteristic is known, atrimming strategy can be determined to provide the correct degree ofadjustment in order to bring the output voltage or current withintighter limits.

[0021] Embodiments of the present invention enable maintaining anaccurate power supply output characteristic without the need for directsensing of the output. Accordingly, one embodiment of the presentinvention provides a technique to trim the switching regulator such thatindirect sensing of the output voltage and current can be employed whilemaintaining the accuracy more normally associated with direct sensingtechniques. When using this type of indirect sensing, power supplies areinfluenced by the manufacturing tolerance of other circuit componentssuch as the transformer and achieving tight tolerances in the outputcurrent and voltage is not usually possible. In one embodiment, improvedoutput voltage or current accuracy is realized by trimming the switchingregulator after the power supply has been assembled. The trimming can beused to adjust either the output voltage or current.

[0022] In sum, embodiments of the present invention introduce severalnovel techniques to achieve circuit trimming after the final powersupply circuit assembly is complete. This allows the power supplycircuit to be tested in its final form and trimming to be performed inresponse to the actual output characteristics of a specific power supplycircuit.

[0023] To illustrate, FIGS. 1 to 3 show several embodiments of theinvention where mechanical trimming is employed. As shown in FIG. 1, aprinted circuit board 101 is illustrated including traces 103, 105, 107and 109. Trace 103 is an unbroken trace. Trace 105 is broken at a brokentrace slot 111 in printed circuit board 101. In one embodiment, brokentrace slot 111 is formed by cutting, sawing, etc. from the edge ofprinted circuit board 101. Trace 107 is broken at nibble location 113 inprinted circuit board 101. As shown, the printed circuit board 101 hasbeen nibbled at the edge of the printed circuit 101 at nibble location113. Trace 109 is broken at a broken trace punched hole 115 in printedcircuit board 101.

[0024]FIG. 2 shows an example embodiment of breaking off a piece of aprinted circuit board in order to break a trace on the printed circuitboard in accordance with the teachings of the present invention. Inparticular, FIG. 2 shows printed circuit board 201 including traces 203and 205. As shown in the depicted embodiment, printed circuit board 201includes a breakable section 207 including a portion of trace 203 thatis designed to be broken off to break trace 203. As shown, a breakablesection of printed circuit 201 including a portion of trace 205 has beenbroken off at location 209 to break trace 205.

[0025]FIG. 3 shows an example embodiment of punching a hole a printedcircuit board in order to break a trace on the printed circuit board inaccordance with the teachings of the present invention. In particular,FIG. 3 shows printed circuit board 301 including traces 303 and 305. Asshown in the depicted embodiment, printed circuit board 301 includes abreakable hole section 307 including a portion of trace 303 that isdesigned to be broken off to break trace 303. As shown, a breakable holesection of printed circuit 301 including a portion of trace 305 has beenbroken off at location 309 to break trace 305.

[0026]FIG. 4 below shows one embodiment of a trimming technique inaccordance with the teachings of the present invention in a layout of apower supply switching regulator electrical circuit with one or moretrim pins. In particular, FIG. 4 shows a printed circuit board 401including traces 403 and an electrical circuit chip 405 mounted orassembled on the printed circuit board. In one embodiment, traces 403are metal traces or include other suitable electrically conductivematerials and electrical circuit chip 405 is power supply regulatorchip. As shown in the depicted embodiment, traces 403 are routed to theedge of the printed circuit board 401 from electrical circuit trimterminals T1, T2, T3 and T4 of chip 405. This allows a slot 405 or thelike to be cut in the edge of printed circuit board 401 to break traces403 as necessary during the trimming process. In the example embodimentshown in FIG. 4, the trim terminals T1, T2, T3 and T4 are coupled to oneor more source terminals S coupled to for example a ground voltageunless the printed circuit board is cut, broken, nibbled or the like tobreak trace 403. In one embodiment, electrical parameters of chip 405are trimmed after chip 405 is mounted or assembled on printed circuitboard 401 and the circuit is assembled by breaking trace 403 as desiredto break the connection from the desired trim terminals T1, T2, T3and/or T4 to ground. Accordingly, trimming can be performed in discreteincrements in accordance with the teachings of the present invention inresponse to which trim terminals T1, T2, T3 and/or T4 are disconnectedfrom ground.

[0027]FIG. 5 shows a block diagram of one embodiment of a switchingregulator or power supply regulator 505 that employs a trimmingtechnique in accordance with the teachings of the present invention. Inone embodiment, power supply regulator 505 is fabricated as a monolithicchip. As shown, power supply regulator includes a power switch 513coupled between a drain terminal 507 and a source terminal 509. Powerswitch 513 is controlled by an oscillator and control circuit 515. Inone embodiment, a start-up circuit 517 is coupled to a drain terminal507, a control input terminal 511 and oscillator and control circuit515. In one embodiment, a current limit circuit is coupled to theoscillator and control circuit 515 and across the power switch 513between the drain terminal 507 and source terminal 509. In oneembodiment, a current sensor is coupled to the control input terminal511, source terminal 509 and oscillator and control circuit 515.

[0028] As illustrated in FIG. 5, power supply regulator 505 includes apower switch 513 coupled between electrical terminals 507 and 509. Inone embodiment, power switch 513 comprises a metal oxide semiconductorfield effect transistor (MOSFET). In one embodiment, power switch 513comprises an n-channel MOSFET having a drain coupled to terminal 507 anda source coupled to terminal 509. In one embodiment, terminal 507 isconfigured to be coupled to a positive input supply rail and terminal509 is configured to be coupled to an energy transfer element of a powersupply.

[0029] As shown in the embodiment depicted, power supply regulator 505also includes a current sensor 521 coupled to receive a current throughcontrol input terminal 511. In one embodiment, the current receivedthrough the control input terminal 511 is responsive to a reflectedvoltage from a energy transfer element of a power supply that powersupply regulator 505 is coupled to regulate. In one embodiment, powerswitch 513 is switched in response to the current received through thecontrol input terminal 511. In addition, current sensor 521 provides inone embodiment a low impedance connection between control input terminal511 and terminal 509. A control circuit 515 is coupled to current sensor521 and power switch 513 in one embodiment. As such, control circuit 515is coupled to control the switching of power switch 513 responsive tothe current coupled to be received through control input terminal 511.

[0030] In one embodiment, oscillator and control circuit 515 includes avoltage mode or a current mode pulse width modulator (PWM) regulator orthe like to control the switching of power switch 513. In anotherembodiment, control circuit 515 includes an on/off control circuit, or avariable frequency circuit, or a cycle skipping circuit, or the like tocontrol the switching of power switch 513.

[0031] In one embodiment, current limit circuit 519 monitors the currentthat flows through power switch 513 when turned on by monitoring thedrain to source voltage of power switch 513. In one embodiment, the onresistance of power switch 513 is used as a current sense resistor. Inone embodiment, when the current that flows through power switch 513reaches a current limit, oscillator and control circuit 515 adjusts theswitching of power switch 513 accordingly such that that the currentthat flows through power switch 513 does not exceed the current limit.

[0032] In one embodiment, the current limit of the power switch 513determined by current limit circuit 519 is adjusted in response to acurrent representative of the reflected voltage received through controlinput terminal 511 as sensed by current sensor 521. For example, in oneembodiment, the current limit is adjusted from a lower value duringstart up of the power supply to a higher value at a regulation currentthreshold.

[0033] In one embodiment, a bias current used to power the circuitry ofpower supply regulator 505 after start-up is also coupled to be receivedthrough control input terminal 511. In one embodiment, a capacitor isconfigured to be coupled between control input terminal 511 and sourceterminal 509. In one embodiment, the capacitor configured to be coupledbetween control input terminal 511 and source terminal 509 is chargedwith a current during start-up with start-up circuit 517.

[0034] As shown in the embodiment of FIG. 5, the power supply regulator505 also includes one or more trim terminals, such as for example T1,T2, T3 and T4, coupled internally to a trim block 523. In oneembodiment, trim block 523 includes sense circuitry 525 coupled to thetrim terminals T1, T2, T3 and T4 and current limit circuit 519,oscillator and control circuit 515 and current sensor 521. In oneembodiment, sense circuitry 525 includes circuitry to sense an impedancefrom each respective trim terminal T1, T2, T3 and/or T4 to a voltagesuch as for example ground.

[0035] In one embodiment, trim block 523 is used internally to adjustone or more electrical parameters of power supply regulator 505 inresponse to sensed impedances from trim terminals T1, T2, T3 and/or T4.As shown in FIG. 5, one embodiment of sense circuitry of sense circuitry525 is coupled to current limit circuit 519, oscillator and controlcircuit 515 and current sensor 521 to trim electrical parametersassociated with current limit circuit 519, oscillator and/or controlcircuit 515 and current sensor 521 in accordance with the teachings ofthe present invention. For example, in one embodiment, trim block 523 isused to trim one or more of the regulation current thresholds associatedwith the control input terminal 511, the current limit in the currentlimit circuit 519 and/or the frequency in the oscillator and controlcircuit 515. In one embodiment, trim block 523 is designed to trim oneor more electrical parameters using the mechanical trimming techniquessuch as for example those described above by breaking traces on aprinted circuit board on which power supply regulator 505. In oneembodiment, the traces may be broken after assembly of the power supplyis complete.

[0036] In another embodiment, or electrical trimming techniques may beemployed to trim power supply regulator 505. In one embodiment,electrical parameters are trimmed by programming for example fuses,anti-fuses or the like included in power supply regulator 505. Toillustrate, in an example embodiment in which power supply regulator 505is trimmed electrically instead of mechanically, trimming is achieved byusing zener zapping. In such an embodiment, power supply regulator 505includes for example includes a respective current source and arespective zener diode coupled to each trim terminal. For example, asdepicted in the embodiment of FIG. 5, current source 527 and zener diode535 are coupled to trim terminal Ti, current source 529 and zener diode537 are coupled to trim terminal T2, current source 531 and zener diode539 are coupled to trim terminal T3 and current source 533 and zenerdiode 541 are coupled to trim terminal T4.

[0037] In one embodiment employing electrical trimming in accordancewith the teachings of the present invention, the zener diodes such asfor example zener diodes 535, 537, 539 and/or 541 prior to zapping orprogramming exhibit a high impedance to circuit common or ground and aretherefore the equivalent of an open circuit in the mechanical schemesdiscussed above. In one embodiment, and internal current sources such asfor example current sources 527, 529, 531 and/or 533 are coupled to therespective cathode of zener diodes 535, 537, 539 or 541 to provide apull up to provide reliable sensing of the zener voltage prior tozapping or programming.

[0038] When a zener diode 535, 537, 539 and/or 541 is correctly zapped,it exhibits low impedance to circuit common (source terminal 509) and istherefore the equivalent of a short circuit to circuit common (sourceterminal 509) in the mechanical schemes discussed above. In oneembodiment, the zapping itself is performed according to thecharacteristics of the zener diodes 535, 537, 539 and/or 541. In oneembodiment, the internal zener voltage is exceeded with a fixed currentapplied to the corresponding trim terminal T1, T2, T3 or T4 for aspecified period sufficient to zap the corresponding zener diode 535,537, 539 or 541. In another embodiment, an external capacitor (notshown) of known value is charged to a known voltage to provide a knownenergy source. When applied to corresponding the trim terminal T1, T2,T3 or T4, the corresponding internal zener diode 535, 537, 539 or 541 iszapped as the external capacitor discharges, dissipating its storedenergy in the respective zener diode 535, 537, 539 or 541.

[0039]FIG. 6 is a schematic of one embodiment of a power supply 601 inaccordance with the teachings of the present invention. Power supply 601includes a power supply regulator 621 similar to power supply regulator505 described in FIG. 5. A shown in the embodiment of FIG. 5, powersupply regulator 621 includes one or more trim terminals T1 . . . 4, adrain terminal 623, a source terminal 629 and a control input terminal625. In one embodiment, operation of these terminals is similar to thatdescribed in connection with the corresponding terminals in FIG. 5. Inone embodiment, the “external capacitor” discussed in connection withFIG. 5 may correspond to capacitor 631 of FIG. 6 coupled betweenterminals 625 and 629 for start up energy storage and supply bypassing.In one embodiment, capacitor 631 also provides control loop compensationfor power supply 601. In another embodiment, the bias current used topower the circuitry of power supply regulator 621 may be derived fromdrain terminal 623. In this embodiment, a capacitor may be coupledbetween a separate bias supply electrical terminal (not shown) andsource terminal 629 for energy storage and high frequency bypassing.

[0040] Operation of an embodiment of power supply 601 employing a powersupply regulator similar to power supply regulator 505 of FIG. 5 forpower supply regulator 621 in FIG. 6 is as follows. Assume for thisillustration that terminals 623, 625 and 629 of power supply regulator621 correspond to terminals 507, 511 and 509, respectively, of powersupply regulator 505. In one embodiment, an alternating current (AC)voltage is applied to AC input 603. In one embodiment, the AC voltagethat is applied to AC input 603 is 85 to 265 volts. Rectifier circuitreceives the AC voltage and applies that rectified voltage to a primarywinding 661 of an energy transfer element 645 and a drain terminal 623of a power supply regulator 621. A regulated direct current (DC) voltageis generated at DC output 655.

[0041] Referring to both FIGS. 5 and 6, at power-up or a beginning of astart-up period of power supply 601, start-up circuit 517 in oneembodiment is coupled to provide a current between terminal 623 andcontrol input terminal 625 to charge capacitor 631 to an adequatevoltage to provide the bias current used to supply power to power supplyregulator 621 for the duration of the start-up condition. In oneembodiment, a current source (not shown) included within start-upcircuit 517 is activated to draw current from terminal 623 to chargecapacitor 631 through control input terminal 625. After capacitor 631 issufficiently charged, the current source in start-up circuit 517 isdeactivated. When the sufficient voltage is reached in capacitor 631,the energy stored in capacitor 631 is used in one embodiment to operatepower supply regulator 621 long enough to complete the start-up of powersupply 601.

[0042] In another embodiment, an additional terminal (not shown) may beincluded for connection to a start-up energy storage capacitor, such asfor example capacitor 631. Alternatively, in this embodiment, the biascurrent used to power the power supply regulator 621 may be derived fromterminal 623 both during start-up and during normal operation afterstart-up. In either case, the capacitor coupled to the additionalterminal can also perform the function of high frequency bypassing.

[0043] During start-up of power supply 601, the current received throughcontrol input terminal 625 representative of the reflected voltage V1657 from primary winding 661 of energy transfer element 645 issubstantially zero. The reflected voltage V1 657 is the voltage acrossthe primary winding when the power switch 531 is off and the energy isbeing delivered to the output. At this time, one embodiment of currentlimit circuit 519 and oscillator and control circuit 515 are coupled toswitch power switch 513 such that a limited amount of power is deliveredto secondary winding 663 of energy transfer element 645 to charge outputcapacitor 651, resulting in reflected voltage V1 657 eventually beinglarge enough to charge capacitor 637 to drive current through resistor639 into control input terminal 625.

[0044] In one embodiment, after start-up, the current driven throughresistor 639 is also used to supply the bias current used to supplypower to power supply regulator 621. In one embodiment, the currentdriven through resistor 639 to supply the bias current also includescurrent resulting from the inductive leakage voltage spikes that occuracross primary winding 661 when power switch 513 is switched off. It isappreciated that known switched mode power supplies often simplydissipate the energy caused by leakage inductance. Thus, power supply601 has increased efficiency over known switched mode power suppliesbecause a part of the energy from the leakage inductance is utilized tosupply power to power supply regulator 621. In addition, a separate biaswinding on the energy transfer element 645 is not needed to provide thebias supply current, as is sometimes the case in known switched modepower supplies. Thus, power supply 601 operates with fewer componentsthan known switched mode power supplies, which reduces cost.

[0045] In one embodiment, as the current representative of the reflectedvoltage V1 657 driven through resistor 639 into control input terminal625 increases, power supply regulator 621 is coupled to increase thepower level delivered to DC output 655 such that a substantiallyconstant output current is delivered by DC output 655, which issubstantially independent of the output voltage across DC output 655. Inone embodiment, the power level delivered to the DC output 655 ischanged by changing the current limit determined by current limitcircuit 519 of power switch 513 from a lower value at start-up as afunction of the current through resistor 639 to a higher value at theregulation current threshold.

[0046] In one embodiment, when the current representative of thereflected voltage V1 657 driven through resistor 639 reaches theregulation current threshold, power supply regulator 621 reduces thepower delivered by power switch 513 such that reflected voltage V1 657is maintained very close to this level, which drives currentapproximately equal to the regulation current threshold through resistor639. Accordingly, the output voltage V2 659 is maintained at a voltagerelated to reflected voltage V1 657 based on the turns ratio of energytransfer element 645, the regulation current threshold value and thevalue of resistor 639.

[0047] It is noted that power supply 601 of FIG. 6 is illustrated withphysical trim connections between terminals T1, T2, T3 and T4 and sourceterminal 629 to enable the mechanical trimming techniques discussedabove. In another embodiment, however, the power supply regulator 621can be designed to also accept electrical trims through the pins T1, T2,T3 and T4 as discussed above.

[0048]FIG. 7 is a diagram 701 illustrating the relationships of outputcurrent and output voltage of several embodiments of a power supply inaccordance with the teachings of the present invention. As illustratedin curve 703 of FIG. 7, one embodiment of a power supply in accordancewith the teachings of the present invention exhibits a substantiallyconstant output current/constant output voltage characteristics. Thatis, as output loading increases, output voltage remains substantiallyconstant until the output current reaches an output current threshold.As the output loading is increased further, the output voltage decreasesas the output current remains substantially constant over the drop inoutput voltage. It is appreciated that the constant outputvoltage/constant output current characteristics of one embodiment of thepresent invention are suitable for battery charger applications or thelike.

[0049] In one embodiment, the output current and output voltagerelationship can be adjusted by trimming the power supply regulator inaccordance with the teachings of the present invention. In oneembodiment, trimming control input terminal current sense of the powersupply regulator adjusts the output voltage, as indicated with referencenumeral 709. In one embodiment, trimming either the oscillator frequencyor the drain current limit of the power supply regulator adjusts theoutput current, as indicated with reference numeral 711.

[0050] In one embodiment, curve 705 shows that one embodiment of a powersupply in accordance with the teachings of the present invention has asubstantially constant voltage/constant current characteristic exceptthat below a certain voltage level the current increases. In yet anotherembodiment, curve 707 shows that another embodiment of a power supply inaccordance with the teachings of the present invention has asubstantially constant voltage/constant current characteristic exceptthat below a certain voltage level the current decreases. In oneembodiment, control circuit 515 in FIG. 5 provides constant outputvoltage control by reducing the duty cycle of power switch 513 whencurrent sensor 521 senses that the current received at control inputterminal 511 has reached the regulation current threshold. In oneembodiment, substantially accurate regulation is provided by powersupply regulator 505 by control circuit 515 causing relatively largeduty cycle changes in power switch 513 for relatively slight changes incurrent sensed by current sensor 521 above the regulation currentthreshold. As a result, the current received through control inputterminal 511 remains substantially constant near the regulation currentthreshold in one embodiment of the present invention.

[0051] In one embodiment, the constant output voltage value of curve 703in FIG. 7 is determined by the value of resistor 639 and the turns ratioof the transformer of energy transfer element 645 in FIG. 6 for a givenregulation current threshold current value. In one embodiment, theconstant output current value of curve 703 in FIG. 7, is determined bythe current limit of power switch 513 at the regulation currentthreshold, the turns ratio of the transformer of energy transfer element645, and the inductance of primary winding 661. It is appreciated thatit is possible to select any combination of output voltage and constantcurrent value within the power range of power supply regulator 621 byselecting an appropriate primary inductance and turns ratio for thetransformer of energy transfer element 645 and the value of resistor639.

[0052] Thus, in one embodiment, constant output voltage/constant outputcurrent characteristics are provided by power supply 601 through sensingof the reflected voltage V1 657. In the embodiments illustrated, flybackconverter power supplies have been provided for explanations of thepresent invention. It is appreciated that other power supplyconfigurations such as for example non-isolated buck converter powersupplies using for example inductors for energy transfer elements mayalso be utilized in accordance with the teachings of the presentinvention. Since the inductor used in the non-isolated buck converterhas only one winding which is coupled to both input and output, theequivalent turns ratio is equal to 1 and the reflected voltage is thesame as the output voltage.

[0053] In one embodiment of a power supply employing the switchingregulator block diagram such as for example that shown in FIG. 5, thetrims on pins T1, T2, T3 and T4 can be used to trim either the switchingfrequency or the regulation current threshold current (of the currentsensor 521) of the power supply regulator or the current limit (of thepower switch 513) of the power supply regulator 505, to influence thepower supply output characteristic.

[0054] The regulated value of V_(OUT) at DC output 655 can therefore beadjusted by using one or more of the trim terminals (T1, T2, T3 and T4)to adjust the regulation current threshold of a current I_(C) flowinginto control input terminal 625. The threshold can be raised (increasingregulation current threshold of current I_(C) and therefore V_(OUT)) orlowered (reducing regulation current threshold of current I_(C) andtherefore V_(OUT)) using the trim terminals T1, T2, T3 and T4.

[0055] In one embodiment, the power supply 601 shown in FIG. 6 canoperate in either the discontinuous or continuous mode. In thediscontinuous mode of operation, the maximum output power of the powersupply 601 is governed by the following equation:

Output Power Max=½L·I _(PK) ² ·f·η  Equation 1

[0056] where L is the primary inductance of the energy transfer element645 of FIG. 6, I_(PK) is the peak primary current limit of power supplyregulator 621, f is the power supply regulator 621 operating frequencyand 11 is the efficiency of power supply 601

[0057] In addition,

Output Power Max=V _(OUT) ·I _(OUTMAX)  Equation 2

[0058] where V_(OUT) is the power supply DC output voltage andI_(OUTMAX) is the maximum power supply DC output current.

[0059] Accordingly, combining Equations 1 and 2,

V _(OUT) ·I _(OUTMAX)=½L·I _(PK) ² ·fη  Equation 3

[0060] It follows from Equation 3 that if V_(OUT) is regulated by thepower supply regulator as described above such that V_(OUT) issubstantially constant, then varying either the current limit oroperating frequency of the power supply regulator will adjust themaximum DC output current of the power supply. These aspects of oneembodiment of the present invention are illustrated in FIG. 7

[0061] The output characteristic of the power supply can therefore beadjusted by trimming: (1) regulation current threshold of I_(C)—toadjust V_(OUT), and (2) either the power supply regulator current limitor the operating frequency to adjust I_(OUT).

[0062] In one embodiment, if the power supply is designed to operate inthe continuous mode of operation at peak power output, the samevariables can be trimmed though the relations of Equations 1 and 3 inaccordance with known techniques.

[0063] By applying the trims as discussed above, the outputcharacteristic can be adjusted as shown in FIG. 7. In one embodiment,trim terminals T1, T2 and T3 are used to adjust the operating frequencyof the power supply regulator with weightings of approximately 4, 8 and16% respectively, which controls the output peak power of the powersupply. In one embodiment, trim terminal T4 is used to trim the I_(C)regulation current threshold of the power supply regulator byapproximately 7.5%, which controls the output voltage of the powersupply.

[0064] In the foregoing detailed description, the method and apparatusof the present invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A method, comprising: assembling one or more components of an electrical circuit onto a printed circuit board having one or more electrical connections coupled to the said one or more components; and trimming an electrical parameter of the electrical circuit, wherein trimming the electrical parameter of the electrical circuit includes removing a portion of the printed circuit board to break the electrical connection on the printed circuit board.
 2. The method of claim 1 wherein removing the portion of the printed circuit board includes cutting a slot in the printed circuit board to break the electrical connection on the printed circuit board.
 3. The method of claim 1 wherein removing the portion of the printed circuit board includes sawing a slot from an edge of the printed circuit board to break the electrical connection on the printed circuit board.
 4. The method of claim 1 wherein cutting the slot in the printed circuit board includes nibbling a slot from an edge of the printed circuit board to break the electrical connection on the printed circuit board.
 5. The method of claim 1 wherein removing the portion of the printed circuit board includes breaking off a piece of the printed circuit board to break the electrical connection on the printed circuit board.
 6. The method of claim 1 wherein removing the portion of the printed circuit board includes punching a hole in the printed circuit board to break the electrical connection on the printed circuit board.
 7. The method of claim 1 wherein the electrical connection on the printed circuit board includes a metal trace on the printed circuit board, wherein the portion of the printed circuit board that is removed includes a portion of the metal trace.
 8. The method of claim 1 further comprising regulating a switched mode power supply with the electrical circuit device.
 9. The method of claim 1 wherein trimming the electrical parameter of the electrical circuit includes trimming a frequency of the electrical circuit.
 10. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes trimming a switching frequency of the switched mode power supply.
 11. The method of claim 1 wherein trimming the electrical parameter of the electrical circuit includes trimming a current limit of the electrical circuit.
 12. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes trimming a current limit of a power switch of the switched mode power supply.
 13. The method of claim 1 wherein trimming the electrical parameter of the electrical circuit includes trimming a regulation current threshold of the electrical circuit.
 14. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes trimming a regulation current threshold that controls a voltage output of the switched mode power supply.
 15. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes trimming a regulation current threshold that determines an onset of a duty cycle control of the switched mode power supply.
 16. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes adjusting an output voltage of the switched mode power supply.
 17. The method of claim 8 wherein trimming the electrical parameter of the electrical circuit includes adjusting an output current characteristic of the switched mode power supply.
 18. A method, comprising: assembling a switched mode power supply; and adjusting an output current characteristic of the power supply in response to trimming a switching frequency of the switched mode power supply after the switched mode power supply is assembled.
 19. The method of claim 18 wherein trimming the switching frequency of the switched mode power supply includes trimming the switching frequency of the switched mode power supply in discrete increments.
 20. The method of claim 18 wherein trimming the switching frequency of the switched mode power supply includes trimming the switching frequency of the switched mode power supply of the switched mode power supply electrically.
 21. The method of claim 18 wherein trimming the switching frequency of the switched mode power supply includes changing a value of current in a constant current region of an output current characteristic of the switched mode power supply.
 22. A method, comprising: assembling a switched mode power supply; and adjusting a value of a constant output current characteristic of the switched mode power supply in response to trimming a current limit of a power switch of the of the switched mode power supply after the switched mode power supply is assembled.
 23. The method of claim 22, wherein trimming the current limit of the power switch of the switched mode power supply includes trimming the current limit of the power switch of the switched mode power supply in discrete increments.
 24. The method of claim 22 wherein trimming the current limit of the power switch of the switched mode power supply includes trimming the current limit of the power switch of the switched mode power supply electrically.
 25. A method, comprising: assembling a switched mode power supply including a switched mode regulating with a control input terminal; sensing at the control input terminal of the switched mode regulator a current that is representative of an output voltage of the switched mode power supply, the control input terminal having a regulation current threshold; and adjusting an output voltage characteristic of the switched mode power supply in response to trimming a value of the regulation current threshold after assembly of the switched mode power supply.
 26. The method of claim 25 wherein trimming value of the regulation current threshold includes trimming value of the regulation current threshold in discrete increments.
 27. The method of claim 25 wherein trimming value of the regulation current threshold includes trimming value of the regulation current threshold electrically.
 28. The method of claim 25 wherein trimming value of the regulation current threshold includes adjusting a voltage level at the output of the switched mode power supply. 